Timing of commitment to autogamy in Paramecium bursaria

Abstract

To elucidate the timing of commitment to autogamy in Paramecium bursaria, cells were treated with an autogamy-inducing agent (1.25% methyl cellulose) for 0, 1, 2, 3, 5, and 24 h, and the duration of treatment required for the induction of autogamy was examined 24-25.5 h after the onset of treatment. Many cells treated for 0-2 h were at the vegetative stage, but many cells treated for 3-24 h were at stages from the third pre-zygotic division to the third post-zygotic division and normally underwent autogamy. Therefore, treatment with methyl cellulose for 3 h or more is considered necessary for the induction of autogamy. However, some of the cells treated for 3 h were at the stage of developing either a swollen and spindle-shaped micronucleus or an early round-shaped micronucleus, which is indicative of the pre-meiotic S phase. This suggests that if cells proceed beyond the stage of an early round-shaped micronucleus, they normally undergo autogamy, but if not, they terminate at the stage of either a swollen and spindle-shaped micronucleus or an early round-shaped micronucleus. Consequently, cells of P. bursaria are committed to autogamy just after the stage of an early round-shaped micronucleus (the pre-meiotic S phase).